Feedback control systems for controlling engine idling speed, which control intake air amount or ignition timing according to a signal based on a difference between a predetermined target idling speed and an actual idling speed, are well known. For example, Japanese Patent Application laid open No. 54-98413 shows such a system. This system includes a feedback control mechanism and a load switch for detecting a load change under idling condition. When the load switch detects increasing load, the system increases a normally constant intake air amount by enlarging the opening area of a by-pass air passage provided in the intake passage during a predetermined time to prevent a decrease in the actual engine speed due to the load.
Such a system which compensates a normally constant intake air amount according to a load change is unable generally to keep a proper engine idling speed for various engine operations conditions. For example, an engine connected with the automatic transmission experiences two different load conditions. One load condition is that the transmission is in a driving range, and another load condition is that the transmission is in a non-driving range, such as a neutral or parking range. When the automatic transmission is in the driving range, the friction load of a gear train of the transmission makes the engine idling speed stable.
On the other hand, when the automatic transmission is in the non-driving range, the engine idling speed is unstable compared with the condition when the automatic transmission is in the driving range because the friction load of the gear train is removed.